It is not a receiver nor a full amplifier. It's a quadraphonic unit (in a way an early predecessor to surround sound). Quadraphonic amplifiers will play 4 seperate channels of audio over 4 speakers instead of the 2 channels you get from your regular stereo. You set up 2 speakers in front of you and two in the back. The unit you've got is meant to be added to a regular stereo amplifier to add the 2 rear channels you need in order to get 4 channel music playback.
The unit was made to match with a Pioneer SA-7100, SA-8100 or SA-9100.
It also has a decoder to play back SQ vinyl. Normal vinyl has just two tracks/ channels of music (as the groove has only two sides to it) but SQ vinyl has 4 tracks of music encoded. The QL-600A will seperate the 2 rear channels from the front channels in order to give you quadraphonic sound.

There is a lot of content to be found on the internet about quadraphonic sound (might you be interested).